The present invention relates to thin film electroluminescent ("EL") devices, and in particular to a method for making EL devices that contain at least one organic light-emitting layer comprising a conjugated polymer prepared by halogen precursor route chemistry. EL devices are structures that emit light when subjected to an applied electric field. EL devices have been conventionally used for a variety of purposes, such as for shaped elements known as light emitting diodes that operate as indicators on vehicle dashboards, cookers, video recorders and the like.
There have been known two layer EL devices comprising two organic layers, wherein the first organic layer comprises an organic light emitting compound and a second organic layer comprises an organic hole transport compound, both layers being layered on each other and arranged between a metal electrode (e.g., cathode) and a transparent electrode (e.g., anode). There have also been known three layer EL devices, in which an organic electron transport layer, an organic fluorescent layer and an organic hole transport layer are layered on each other and arranged between a metal electrode (e.g., cathode) and a transparent electrode (e.g., anode). In such three layer EL devices, the organic hole transport layer has the functions of transporting holes injected from the anode and blocking electrons, while the organic electron transport layer has a function of transporting electrons injected from a cathode. The light emitting layer can further include a fluorescent material capable of emitting light in response to hole and electron recombination. Such two and three layer structures are typically manufactured by applying the organic layers by well known solvent coating techniques for polymeric materials or by vacuum deposition for molecular systems.
U.S. Pat. No. 5,276,381 to Wakimoto discloses an organic EL device that includes an organic light emitting layer comprising a quinoline derivative and a quinacridone or quinazoline compound. U.S. Pat. No. 4,769,292 to Tang teaches a method for applying EL layers that includes applying a luminescent layer to a substrate by vacuum deposition. U.S. Pat. No. 4,950,950 to Perry discloses a multilayer EL device comprising silane hole transporting agents. U.S. Pat. No. 4,356,429 to Tang illustrates organic EL cells having a hole injecting porphyrinic zone. Vincett, P. S., et al., Thin Solid Films, 94:171 (1982); R. H. Partridge, Polymer, 24:755 (1983); Burroughes, J. H., et al., Nature, 347:539 (1990); Braun, D., et al., Applied Physics Letters, 58:1982 (1991); Braun, D., et al., J. Electronic Materials, 20:945 (1991); Brown, A. R., et al., Applied Physics Letters, 61:2793 (1992); and Kido, J., et al., Applied Physics Letters, 59:2760 (1991) each disclose other organic EL compositions. Recently, thin film devices such as photodetectors (Yu, G., et al., Applied Physics Letters, 64:1540-1542 (1994) and Applied Physics Letters, 64:3422-3424 (1994)), photovoltaic cells (Antoniadis, H., et al., Synthetic Metals, 62:265-271 (1994) and Marks, R. N. Journal of Physics: Condensed Matter, 6:1379-1394 (1994)), field effect transistors (Fuchigami, H., et al, Applied Physics Letters, 63: 1372-1374), and photogeneration layers in photoreceptors (Antoniadis, H., et al., Applied Physics Letters, 62:3167-3169 (1993)) based on PPV and related polymers, have also been reported.
There have also been known EL devices wherein the light emitting layer comprises a conjugated polymer. There are two important approaches to the preparation of conjugated polymer thin films, namely the precursor approach and side chain approach. The former relies on the preparation of a soluble precursor polymer that can be cast into thin films. For instance, WO 90/13148 to Friend discloses an EL device comprising a semiconductive light emitting layer made of a conjugated polymer known as poly(p-phenylene vinylene) ("PPV"). The disclosed PPV is prepared via sulphonium precursor route chemistry using sulphonium precursors that are soluble in water and methanol.
The disclosed soluble sulphonium precursor is deposited on electroded substrates such as indium tin oxide (ITO) coated glass and subsequently thermally converted to form PPV that emits light in response to an applied electric field. In particular, a sulphonium salt precursor is transformed to a final conjugated polymer film through solid state thermo- or photo-conversion.
The sulphonium precursor route involves the polymerization of p-xylene-bis-(tetrahydrothiopheniumchloride), or one of its analogs or derivatives, in the presence of a base in water or methanol to give the corresponding sulphonium precursor polymer. After purification, the sulphonium precursor polymer solution is used to cast films that are then thermally converted to give PPV thin films. Due to the solubility of sulphonium precursors, the sulphonium precursor route is useful for the preparation of PPV. The use of sulphonium precursors to obtain many other electroluminescent substituted PPV derivatives, however, is disadvantageous for several reasons. For instance, the sulphonium salt precursor route: uses precursor polymers that have an offensive odor; is limited by the number of workable sulphonium salt precursors that can be readily synthesized; produces precursor polymers that are highly unstable; produces polymer precursors that have limited storability properties because they precipitate out of solution as a gel within two weeks when stored in polar solvents, such as methanol; releases environmentally unsafe sulfides and hydrogen halides during thermoconversion; and requires exhaustive and tedious purification steps via dialysis, which in turn lead to low yields of the desired PPV (usually 20%-40% retention). Moreover, depending on the particular sulphonium salt precursor monomer used, polymerization conditions must be modified or adjusted by trial and error to obtain substituted derivatives of PPV that can be used to produce reliable EL devices.
The side chain approach involves the polymerization of a highly substituted monomer to a soluble conjugated polymer that can be cast into thin films directly without conversion. The polymerization of bis(halomethyl)benzenes in the presence of large excess base to poly(arylene vinylenes) is conventionally known as the Gilch synthesis route. See Gilch, et al., Journal of Polymer Science: Part A-1, 4:1337 (1966). In principle, the adaptation of the Gilch route to the polymerization of a bis(halomethyl)benzene with side groups or solubilizing groups should give a soluble poly(arylene vinylene). Unfortunately, this is not the case in practice because of polymer product precipitation during polymerization. Such precipitation is caused by the high molecular weight and/or semicrystallinity of the product. As a result, soluble product is obtained in low molecular weights and very low yields (e.g., 0-10%).
The use of halogen precursors for the production of PPV and substitutes thereof was first reported by Swatos, W. J. et al., Polymer Preprints, 30(1):505-506 (1990) for the preparation of poly(2,5-dihexyloxy-1,4-phenylene vinylene) and then by Hsieh, B., et al., "A Dehydrochlorination (DHCL) Route to Poly(2,3-diphenyl-1,4-phenylene vinylene) (DP-PPV)," Polymer Preprints, 34(2):410-411 (1993). However, the use of substituted PPV derivatives obtained using halogen precursor polymers in EL devices has not been heretofore known or suggested. Such substituted PPV derivatives have not been shown to possess electroluminescent properties as thin films.